The freezing of food using a liquid refrigerant, such as liquid nitrogen, has been practiced on a commercial scale for several years. A typical commercial freezing process begins by placing the food onto a continuous conveyor. The conveyor travels into a bath of the liquid refrigerant in a manner which immerses the food in the liquid refrigerant. The frozen food is removed from the bath by the conveyor and sent for further freezing, processing and packaging.
Processes by which food is immersed in a relatively fixed bath of liquid refrigerant are disadvantageous because the food product in the form of individual pieces of food tend to be frozen together. This requires a mechanical separation procedure which can damage the food, particularly delicate food products such as shrimp, scallops, clams and the like.
Such processes are also disadvantageous because large loses of the liquid refrigerant result from the passage of the conveyor into and out of the bath. The conveyor is continuously being cooled upon entry into the bath and warmed upon exiting the bath. As a result, large quantities of the liquid refrigerant are consumed in the process adding significantly to the cost of freezing.
Another method of freezing food employs a tunnel freezer. The food product is placed onto a continuous conveyor which enters a thermally insulated box equipped with fans. Liquid refrigerant is injected into the insulated box and vaporizes thereby cooling the interior of the insulated box. The fans within the insulated box circulate the cold vapor and convectively freeze the food.
Tunnel freezers of the type described above are disadvantageous because the convective cooling process is relatively slow. In addition, a relatively large insulated box is needed, increasing the cost of the freezer and occupying valuable processing area.
Tunnel freezers are also disadvantageous because like the immersion freezers described above, if the pieces of food are not separated prior to entering the freezer, they must be mechanically separated after the freezing operation. In addition, wet and delicate food products such as shrimp, scallops, clams and the like tend to stick to the conveyor, requiring mechanical removal thereof. The implementation of mechanical devices to separate pieces of food from each other and/or from the conveyor may cause damage to delicate food products.
More recently, methods of freezing have been disclosed, particularly for freezing liquids such as creams, liquid egg and the like, in which a liquid refrigerant flows through inclined channels provided by a trough.
Peter H. Gibson et al., U.S. Pat. No. 4,479,363 disclose a process for freezing a liquid in which the liquid is passed into or onto a stream of liquified gas along an inclined channel. The stream of liquified refrigerant is characterized by a laminar flow and the patent states that turbulent flow is to be avoided.
Peter H. Gibson, U.S. Pat. No. 4,843,840 discloses a process for freezing liquid food products using a channeled conduit to convey the liquid refrigerant. A smooth, non-turbulent flow of the liquid refrigerant is required to achieve uniformity in and control over both the size of product and the extent to which the food product is cooled.
Despite the benefit achieved by these methods, there is still the need for processes of freezing food which at least substantially reduce the freezing of solid pieces of food together and which freezes the food product in an even more cost efficient manner.